OG-53

53. Two decades after the Emerald River Dam was built, none of the eight fish species native
to the Emerald River was still reproducing adequately in the river below the dam. Since the
dam reduced the annual range of water temperature in the river below the dam from 50
degrees to 6 degrees, scientists have hypothesized that sharply rising water temperatures
must be involved in signaling the native species to begin the reproductive cycle.
Which of the following statements, if true, would most strengthen the scientists’ hypothesis?
(A) The native fish species were still able to reproduce only in side streams of the river below the
dam where the annual temperature range remains approximately 50 degrees.
(B) Before the dam was built, the Emerald River annually overflowed its banks, creating
backwaters that were critical breeding areas for the native species of fish.
(C) The lowest recorded temperature of the Emerald River before the dam was built was 34
degrees, whereas the lowest recorded temperature of the river after the dam was built
has been 43 degrees.
(D)Nonnative species of fish, introduced into the Emerald River after the dam was built, have
begun competing with the declining native fish species for food and space.
(E) Five of the fish species native to the Emerald River are not native to any other river in
North America.
53.
For the hypothesis to be tenable it is important that the fish in streams in the Emerald River
area that retain a wide temperature difference have not lost their ability to reproduce. Choice A
asserts that these fish could still reproduce and is thus the best answer.
Choice B undermines the hypothesis by suggesting a completely different hypothesis; choice
C tends to support the claim that the temperature variation has lessened but does not show
that this is the right explanation; since D relates a development after the native species began
to decline, it does not bear on the hypothesis, which concerns the decline’s original cause; and choice E emphasizes the seriousness of the problem but sheds no light on what causes it.

为什么是A？怎么解释？

偶一开始做对是因为把signal理解成“妨碍”了，稀里糊涂地选了A，一看答案才发现不是我想得那样，结果就看不懂了

条件：建大坝后温差小了（50度减至6度），鱼没了

结论：是温差小是鱼少了的原因

加强方法：温差不小的地方（side stream）鱼就没有减少，用一个反面事实加强了结论

谢谢啊！绕来绕去都晕了

另外，搞不懂那个因果关系：“因为大坝减少了温差，所以温度上升signal鱼开始产卵”？？？

很显然，那些鱼喜欢大的温差（就像没建大坝时一样），那么温度上升又怎么说明鱼开始产卵呢？

请NN们指教！

原文意思：某某河建了DAM，使DAM的下游温度发生变化，从而导致NATIVE（即不是再放进去的）的鱼不能繁殖。现在下游温度又变回来了。所以科学家假设温度变回来意味着下游的NATIVE的鱼又要开始繁殖了。

A说，（下游的主流的NATIVE的鱼虽然不能繁殖），但支流的鱼还能繁殖。（如果支流的鱼也不能繁殖，则所有的鱼都不能繁殖，则即使温度变回来，也没有能繁殖的NATIVE的鱼去繁殖）。

Sorry, 不是挑战我心中的逻辑之神

但上面的翻译有些问题, 应为:
科学家推论: 急遽上升的温度一定和signal 原生的鱼开始繁殖有关

我以(C)选项提供温差数据和题目的数据做一个说明:
原始温差: 50度, 最高温: 50+34=84度, 最低温: 34度

水库建后温差: 6度, 最高温:  6+43=49度, 最低温: 43度

因此水温的变化原始是 34~84度

    水库建后水温变化是 43~49度  

        
因此科学家推论: 急遽上升的温度(例如从34度跳到84度)一定和signal 原生的鱼开始繁殖有关!

  如果有不对之处, 还请指正